The present invention relates in general to an optical device, and is concerned, more particularly, with an integral surface plasmon coupler for use in efficiently coupling between free space photons and surface plasmons in the far-infrared and submillimeter spectral regions.
It is well known that in the visible region of the electromagnetic spectrum it is possible to couple essentially all of the bulk electromagnetic wave incident to a metal surface into a surface electromagnetic wave (SEW) or surface plasmon wave within the metal. (For a detailed discussion of surface plasmons see, for example, E. Burstein et al., J. Vac. Sci. Technol., Vol. 11, No. 6, pp. 1003-1019, 1974.)
However, as the wavelength of the incident radiation increases into the infrared region of the spectrum and beyond, efficient coupling between the incident radiation and the surface plasmon becomes more difficult. This is due to increasing delocalization of the electromagnetic field away from the interface and the corresponding decrease in the proportion of the field within the metal as wavelength increases.
Z. Schlesinger, et al, have discussed in Solid State Commun. Vol. 38, pp. 1035-1039, 1981, the phenomenon of coupling between far-infrared radiation and surface plasmons in germanium coated gold or lead surfaces. In a doctoral dissertation submitted to the University of Missouri at Rolla in 1975 and entitled, "Excitation of Surface Electromagnetic Waves at Microwave Frequencies Using Optical Techniques", M. Davarpanah has described the coupling of microwave radiation into a coated metal surface in devices which employ metal rods placed over the coating.
Up to the present time, a number of techniques have been experimented with, but generally resulting in poor coupling efficiency between the incident radiation and the surface plasmon wave. Some of the techniques involve the use of prisms, gratings and edge couplers. As a matter of fact, recently there has been substantial success with a dielectric overcoated grating coupler. See copending application Ser. No. 474,222 filed Mar. 11, 1983. However, one disadvantage with the grating type coupler is that the coupler must be formed into the metal surface on which the surface plasmon is to propagate.
As mentioned previously, prisms have been used in the past in an attempt to provide coupling of electromagnetic radiation with surface plasmon waves on a metal. In the known technique, a prism is supported by a support means so as to be spaced by an air gap from the metal substrate to which the coupling is to take place. This prior art prism arrangement provides for only marginal coupling and is generally difficult to set up, usually requiring some adjustment between the prism and the metal substrate.